1. Field of the Invention
The present invention relates to electrophotographic apparatus and more specifically to apparatus for sensing and controlling the surface charge on an electrostatic printing/copying device.
2. Description of the Prior Art
Copying apparatus utilizing an electrophotographic drum for reproduction have, in the past, employed sophisticated switching devices to control the charging and discharging of the drum surface. Gas lasers used in such configuration utilize relatively expensive optical switching modulators for this purpose. As the speed requirement increases, such apparatus becomes non-cost effective as a result of the high cost of the controls required.
In contrast to this, the solid state infrared laser diode is relatively inexpensive, small in size and readily, rapidly switched on and off without the necessity for costly and sophisticated optical control assemblies.
Infrared sensitive surfaced members have a relatively rapid charge decay cycle, and thus, it is essential that the desired charge placed on the copy surface be and remain fixed at the required potential level for proper density, line resolution and readability of copy.
Prior art apparatus employ, among other things, photo-optical copy density feedback monitoring and measuring systems; vibrating reed electrostatic sensors driven by the coil of an electromagnet, etc. None of these perform completely efficiently or satisfactorily in the contaminated atmosphere of the toner station.
In addition, although many electrostatic charge sensing and measuring techniques are known, none are completely adaptable for the present purposes. For example, U.S. Pat. No. 3,525,936 to R. E. Vosteen describes and illustrates an electrostatic voltmeter wherein a detector preamplifier probe includes a rotatable perforated chopper disc the apertures of which are aligned with a single relatively small opening adjacent to the surface to be monitored. The probe also includes a magnetic reference pickup and an electrostatic detector which "looks" through the apertures to the surface being measured.
U.S. Pat. No. 2,980,855 to C. B. Moore relates to a coulometer used for measuring electrostatic charge on moving nonconductors such as yarn-thread. The patent describes a chopper (e.g., a vertically rotatable member) interposed between a sensing and a reference electrode provided with windows one of a pair of which cyclically exposes the sensing electrode to view of the reference electrode and the other of which simultaneously exposes the sensing electrode solely to the object under evaluation.
U.S. Pat. No. 3,812,419 to H. M. Kaunzinger et al relates to an electronic field mill wherein a probe is positioned beneath an electronic aperture having orthogonally arranged grid electrodes which are attached to an enclosure that is mounted within but separated or insulated from the edge of a shell of a reference surface.
U.S. Pat. No. 4,149,119 to R. F. Buchheit relates to an electrostatic voltmeter wherein a sensing probe is insulatingly mounted within a conductive housing. A rotating vane or shutter is located in front of a window in the housing with the housing in spaced relationship with a test surface. A photo-cell LED is used to sense the frequency of angular rotation of the shutter.
U.S. Pat. No. 4,205,267 to B. T. Williams relates to a non-contacting voltage follower which comprises a probe including a housing containing two electrodes adjacent to an opening in the probe to expose the two electrodes to the field to be measured and means for vibrating the electrodes relative to the opening to vary the compacitive coupling as a function of the rate or vibration.
U.S. Pat. No. 4,267,511 to K. Suzuki et al relates to a surface potential measuring device in which a measuring electrode is disposed adjacent to a surface to be measured. A rotatable chopper carrying a magnet rotates between a Hall effect device and a measuring electrode. The Hall effect device puts out a signal by being subjected to the field action from the magnet each time the chopper passes through the space between the surface to be measured and the measuring electrode.
It is noted that while each of these apparatus provide means for electrostatic field measurement no one or more show or describe the novel arrangement called for by the claims of the present application.
U.S. Pat. Nos. 3,525,936, 4,167,511 and 4,149,119 deal with rotating discs operable in a vertical plane as disclosed. U.S. Pat. No. 4,205,267 deals with vibrating vanes likewise in a vertical mode. U.S. Pat. No. 2,980,855 shows a vertically rotatable, vertically apertured cylinder facing the surface being monitored. U.S. Pat. No. 3,812,419 relates to a static multi-apertured device fixed to the body being monitored. All of these devices are physically quite different from the present invention and in so far as can be determined not adaptable to the present use.